feat: Use float instead of int for GCE_METADATA_TIMEOUT

[ludoch]

Move the logic to read GCE_METADATA_TIMEOUT env variable so we can better control it in app_identity component.

Related internal change is 607513254.

[conventional-commit-lint-gcf[bot]]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot https://conventionalcommits.org/

[estrellis]

The change LGTM